1. Field of the Invention
The invention relates to a cathode ray tube having (1) an electron source having a cathode for emission of electrons, (2) an electron beam guidance cavity having an input and an output for concentrating electrons emitted from the cathode, (3) a first electrode being connectable to a first power supply for applying, in operation, an electric field with a first field strength E I between the cathode and the output of the cavity so as to allow electron transport through the electron beam guidance cavity, and (4) an accelerating grid for accelerating the electrons leaving the cavity and a main electron lens for focusing the accelerated electrons on a display screen.
Such a cathode ray tube may be used in television displays, computer monitors and projection TVs.
2. Description of the Related Art
A cathode ray tube of the type described in the opening paragraph is known from U.S. Pat. No. 5,270,611. This patent describes a cathode ray tube that is provided with the cathode, the electron beam guidance cavity and the first electrode which is connectable to a first power supply for applying the electric field with a first field strength E1 between the cathode and the output aperture. The electron beam guidance cavity has walls in which, for example, a part of the wall near the output has an insulating material having a secondary emission coefficient xcex41. Furthermore, the secondary emission coefficient xcex41 and the first field strength E1 have values that allow electron transport through the electron beam guidance cavity. The electron transport within the cavity is possible when a sufficiently strong electric field is applied in a longitudinal direction of the electron beam guidance cavity. The value of this field depends on the type of material and on the geometry and sizes of the walls of the cavity. The electron transport then takes place via a secondary emission process so that, for each electron impinging on a cavity wall, one electron is emitted on average. The circumstances can be chosen to be such that as many electrons enter the input aperture of the electron beam guidance cavity as will leave the output aperture. When the output aperture is much smaller than the input aperture, an electron compressor is formed which concentrates the luminosity of the electron source by a factor of, for example, 100 to 1000. An electron source with a high current density can thus be made. The accelerating grid accelerates electrons leaving the cavity towards the main electron lens. The main electron lens images the exit hole of the cavity on the display screen and, via a deflection unit, a raster image is formed on the display screen of the tube.
The spot size of the electron beam determines the resolution of the tube. Especially for computer monitor tubes and also television picture tubes, the resolution may be an important feature.
It is, inter alia, an object of the invention to provide a cathode ray tube in which the spot size of the electron beam on the display screen is reduced. This object is achieved by the cathode ray tube according to the invention, which is characterized in that the cathode ray tube comprises a further electron lens between the cavity and the main lens for adapting the diameter of the electron beam to the entrance of the main lens, said further electron lens comprising the first electrode and the accelerating grid. The electron beam entering the main lens is then less divergent and the spherical aberrations caused by the main lens are reduced. The invention is based on the recognition that the electrons leaving the electron beam cavity have a relatively high velocity compared to electrons leaving a conventional cathode, and therefore the diameter of the electron beam entering the main lens is too large. With the prefocussing effect of the further electron lens between the electron beam cavity and the main lens, and given a fixed relationship of the distances between the cathode, main lens and display screen, the diameter of the electron beam entering the main lens can be optimized for a small spot size and minimal spherical aberrations.
Further advantageous embodiments of the invention are defined in the dependent claims.
A particular version of the cathode ray tube according to the invention is characterized in that the first electrode comprises a first and a second part, placed behind each other along an axis of the main lens, the diameter of the first part being smaller than the diameter of the second part. A so-called cup lens is then formed for prefocussing the electron beam before entrance into the main lens. An advantage of the cup lens is its economic design. Moreover, the cup-lens is robust against flashes which occur during the manufacturing process of the cathode ray tube or during operation. The first and second parts may have different symmetric shapes. The shape of the parts can also be adapted in order to reduce astigmatism of the spot on the display screen, for example, the shape of the cup lens may be a rectangle or ellipsoid.
A further version of the cathode ray tube according to the invention is characterized in that the further electron lens further comprises a second electrode which is concentric with the first electrode, the second electrode being connectable to a second power supply for applying, in operation, an electric field with a second field strength E2 between the first and the second electrodes, the voltage of the second power supply being lower than that of the first power supply. An electron lens is then formed having a special shape for prefocusing the electron beam in the entrance of the main lens. An advantage of this electron lens is that some of the electron lens characteristics can be adjusted when the cathode is mounted in the cathode ray tube. This is in contrast with the above-mentioned cup lens, which has characteristics that are completely determined when the cathode is mounted in the cathode ray tube. Furthermore, the first and second electrodes may have a symmetrical shape.
A further version of the cathode ray tube according to the invention is characterized in that the first and second electrodes are substantially in the same plane. A planar electron lens is thus obtained. These planar lenses can be easily made by removing parts of metal forming the first electrode. Furthermore, a planar lens design allows a large degree of freedom in the prefocussing characteristics of the electron lens.
A further version of the cathode ray tube according to the invention is characterized in that the cathode ray tube comprises a third electrode placed between the cathode and the cavity, said third electrode being connectable to a third power supply for applying, in operation, an electric field with a third field strength E3 between the cathode and the third electrode for controlling the emission of electrons. In this way, relatively small modulation voltages can be applied for modulating the electron beam. For example, when the distance between the cathode and the third electrode amounts to 100 micrometers, an amplitude modulation of 5 Volts is sufficient for modulating a current between 0 and 3 mA when conventional oxide cathodes are used. This modulation gauze is described in the unpublished EP patent application 9920199.6.